1. Field of the Invention
The present invention relates to an optical head apparatus and an optical information recording or reproducing apparatus for recording or reproducing information with respect to several types of optical recording mediums having different standards.
2. Description of Related Art
A recording density in an optical information recording or reproducing apparatus is inversely proportional to a square of a diameter of a condensed spot formed on an optical recording medium by an optical head apparatus. That is, the smaller the diameter of the condensed spot is, the higher the recording density becomes. The diameter of the light convergence spot is proportional to a wavelength of a light source in the optical head apparatus, and is inversely proportional to a numerical aperture of the objective lens. That is, when the wavelength of the light source is shorter, and the numerical aperture of the objective lens is greater, the diameter of the condensed spot is reduced. According to the Compact Disk (CD) standard having a capacity of 650 MB, the wavelength of the light source is about 780 nm and the numerical aperture of the objective lens is 0.45. According to the Digital Versatile Disk (DVD) standard having a capacity of 4.7 GB, the wavelength of the light source is about 660 nm and the numerical aperture of the objective lens is 0.6.
To further improve the recording density, hence, a next-generation standard has been proposed or practiced in recent years with a light source having a much shorter wavelength and with an objective lens having a much grater numerical aperture. For example, according to an Advanced optical Disk (AOD) standard for the capacity of 20 GB, the wavelength of the light source is about 400 nm and the numerical aperture of the objective lens is 0.65. According to a Blue Ray Disk (BRD) standard for the capacity of 23.3 GB, the wavelength of the light source is about 400 nm and the numerical aperture of the objective lens is 0.85.
From these backgrounds, there have been demands for an optical head apparatus and optical information recording or reproducing apparatus which have a good compatibility and are capable of recording or reproduction data on/from plural kinds of disks based on different standards. An optical head apparatus capable of recording or reproducing data on/from disks based on any of the DVD and CD standards has already been put to practical use. In addition, another optical head apparatus capable of recording or reproducing data on/from disks based on any of the next-generation standard, DVD and CD standards has been proposed.
An optical head apparatus described in JP-A-2001-43559 is an example of a conventional optical head apparatus capable of recording or reproducing data on/from disks based on any of the next-generation standard, DVD and CD standards. FIG. 83 schematically shows the structure of the optical head apparatus. Modules 311a, 311b, and 311c each comprise a semiconductor laser, a photodetector, and a hologram optical element. The hologram optical element transmits part of light emitted from the semiconductor laser and guide the part of light to a disk. The element further diffracts reflection light from the disk, to guide the reflection light to the photodetector. The semiconductor lasers of the modules 311a, 311b, and 311c have wavelengths of 780 nm, 660 nm, and 400 nm, respectively. A beam splitter 312a transmits the light having wavelengths of 400 nm and 660 nm but reflects the light having the wavelength of 780 nm. Also, the beam splitter 312b transmits the light having wavelength of 400 nm but reflects the light having the wavelength of 660 nm.
Light emitted from the semiconductor laser from the module 311c passes through the beam splitters 312a and 312b and is reflected by a mirror 313. The light is then converged onto the disk 315 based on the next-generation standard by the objective lens 314. Reflection light from the disk 315 passes through the objective lens 314 in a reverse direction, and is reflected by the mirror 313. This light then passes through the beam splitters 312a and 312b and is received by the photodetector in the module 311c. 
Light emitted from the semiconductor laser in the module 311b is reflected by the beam splitter 312b and passes through the beam splitter 312a. This light is then reflected by the mirror 313 and is converged onto the disk 315 based on the DVD standard by the objective lens 314. Reflection light from the disk 315 passes through the objective lens in a reverse direction, and is reflected by the mirror 313. This light then passes through the beam splitter 312a, is then reflected by the beam splitter 312b, and is received by the photodetector in the module 311b. 
Light emitted from the semiconductor laser in the module 311a is reflected by the beam splitter 312a and reflected by the mirror 313. The reflection light is converged onto the disk 315 based on the CD standard by the objective lens 314. Reflection light form the disk 315 passes through the objective lens 314 in a reverse direction and is reflected by the mirror 313. The reflection light is then reflected by the beam splitter 312a and is received by the photodetector in the module 311a. 
On the other side, an example of a conventional optical head apparatus capable of recording or reproducing data on/from disks based on any of the DVD standard and CD standard is an optical head apparatus described in JP-A-2003-123305. FIG. 84 schematically shows the structure of this optical head apparatus. The wavelengths of semiconductor lasers 321a and 321b are 780 nm and 680 nm, respectively. A beam splitter 322a transmits almost all of both of P-polarized and S-polarized components with respect to light having a wavelength of 660 nm. This beam splitter 322a transmits about 25% of each of P-polarized and S-polarized components and reflects about 75% thereof, with respect to light having a wavelength of 780 nm. Another beam splitter 322b transmits almost all of the P-polarized component with respect to light having a wavelength of 660 nm and reflects almost all of the S-polarized component thereof. This beam splitter 322b transmits almost all of both of the P-polarized and S-polarized components, with respect to light having a wavelength of 780 nm.
Light emitted from the semiconductor laser 321b enters as an S-polarized component into the beam splitter 322b. Almost all of the light is reflected, passes through the beam splitter 332a, and is reflected by a mirror 324, and then is transformed from linearly polarized light into circularly polarized light by a wavelength plate 325, and converged onto a disk 327 based on the DVD standard by an objective lens 326. Reflection light from the disk 327 passes through the objective lens 326 in a reverse direction, and is transformed from the circularly polarized light into linearly polarized light which has a polarization direction at right angles to that of the linearly polarized light approaching in its way toward the disk, by the wavelength plate 325. The linearly polarized light is reflected by the mirror 324 and almost all light passes through the beam splitter 322a, then enters as a P-polarized component into the beam splitter 322a. The beam splitter 322a transmits almost all of light. Then a photodetector 323 receives the linearly polarized light.
Light emitted from the semiconductor laser 321a enters as an S-polarized component into the beam splitter 322a. About 75% of the light is reflected, is reflected by the mirror 324, and is transformed from linearly polarized light into circularly polarized light by the wavelength plate 325, and the converged onto the disk 327 based on the CD standard by the objective lens 326. Reflection light from the disk 327 passes through the objective lens 326 in a reverse direction, and is transformed from the circularly polarized light into linearly polarized light which has a polarization direction at right angles to that of the linearly polarized light approaching in its way toward the disk, by the wavelength plate 325. The linearly polarized light is reflected by the mirror 324 and enters as a P-polarized component into the beam splitter 322a. About 25% of the light transmits through the beam splitter 322a. Almost all of the penetrating light passes through the beam splitter 322b and is received by the photodetector 323.
In the conventional optical head apparatus shown in FIG. 83, all of the light having wavelengths of 400 nm, 660 nm, and 780 nm causes loss of the light amount when the light passes through a hologram optical element in a module, in the forward path of the light toward the disk, and when the light is diffracted by the hologram optical element, in the backward path of the light. Under a condition that the product of the penetration rate in the forward path and the diffraction efficiency in the backward path is maximized, the former rate is only 50% and the latter efficiency is only 40.5%. Loss of the light amount in the forward path causes a reduction in the light output during recording. Loss of the light amount in the backward path causes a reduction of the S/N ratio during reproducing. Disks of the next-generation standard have no margins for the light output during recording or the S/N ratio during reproducing Therefore, this is a serious problem. Similarly, disks of the DVD standard have no margins for the light output during recording or the S/N ratio during reproducing. This can also be a serious problem for the disks of the DVD standard. However, disks of the CD standard have margins for both the light output during recording or the S/N ratio during reproducing. Therefore, this cannot be a serious problem for the disks of the CD standard.
On the other aide, in the other conventional optical head apparatus shown in FIG. 84, the light amount of light having a wavelength of 660 nm is not substantially lost when the light is reflected by the beam splitter 322b or when the light passes through the beam splitter 322a, in the forward path of light toward the disk. In the backward path of light, the light having the wavelength of 660 nm is not substantially lost when the light passes through the beam splitter 322a or 322b. Therefore, with respect to disks of the DVD standard, a high light output is obtained during recording and a high S/N ratio is obtained during reproducing. Light having a wavelength of 780 nm is reflected at a predetermined rate by the beam splitter 322a in the forward path, substantially independently from the polarization state. In the backward path, the light having a wavelength of 780 nm passes through the beam splitters 322a and 322b at a predetermined rate, substantially independently from the polarization state. Therefore, with respect to disks of the CD standard, the birefringence of the disk varies, so that the light amount received by the photodetector does not substantially vary even when the polarization state of the reflection light from the disk varies.
In some kinds of optical system of the optical head apparatus, there is a case that the light amount received by the photodetector varies due to changes of the birefringence of the disk. If the light amount is too small, a sufficient S/N ratio cannot be obtained in a circuit in a rear stage. On the contrary, if the light amount is too great, the circuit in the rear stage is saturated. With respect to disks of the CD standard, the birefringence of the disk varies greatly and thereby causes a serious problem. In the conventional optical head apparatus shown in FIG. 84, however, this problem has been solved. With respect to disks of the DVD standard, changes of the birefringence of the disk cannot be said to be sufficiently small and therefore can be a serious problem. With respect to disks of the next-generation standard, changes of the birefringence of the disk are small and therefore cannot be a serious problem. However, in the conventional optical head apparatus shown in FIG. 84, recording or reproducing cannot be performed with respect to disks of the next-generation standard.